I was wondering if I were to plant a seedling in one end of a rectangular container, as pictured below, would it affect root development, or would the plant develop roots where there is space?
Yes it would effect the root development as the plant grows out of the seedling stage.
Roots develop to best serve the plants needs depending on its given location based on basic rules.
if the growing medium keeps drying on the outside edges of the rectangle pot and the centre is kept moist, then the roots will go towards the water, which happens to be lower down and will simultaneously stay where the food is as it grows, if that makes sense.
Seedlings dont reach out too far they are more concerned with finding local nutrients with in reach as well as growing towards the regularly moistened area.
So the environment will definitely have an effect on the choices roots make, But plants have had a long time to develop strategies and methods to ensure success.
With certain plants like citrus, if the rootball is not uniformly watered (waterd only on one side) it could lead to splitting of the fruit!
Yes, it may well affect root development. Planting a seedling in such a large container on its own isn't a great idea, because it may put out long, meandering roots rather than forming a good rootball. That leaves a lot of unoccupied soil in the container which you then have to keep wet or damp in order to keep your little plant alive, and that can lead to 'souring' of the soil These are the reasons for potting up something in a small pot first, then potting on as the plant and its rootball get bigger.
Over the longer term, if the seedling survived, it depends what plant it is as to whether it would take up all the available root space. It's not clear what the depth of your container is, but if the seedling was a tree seedling, it would need a deep, narrower pot because many trees develop a tap root quite early on.
It would first depend on the type of seedling. If it's a plant that only grows small roots in a fairly confined area then it may not be affected at all or very little. If it's a tree or a large plant, going to develop roots in every available area in your small space. So, I think the answer you are looking for is, the plant will develop roots where there is space. If the entire space it too confined for the plant the plant will suffer.
There are other factors, if the seedling is bound in a dense soil when planted it may not develop proper roots at all. If water and nutrients are only provided near the plant, leaving the rest of the soil fairly barren then the seedling has no reason to spread or no ability to leave its confined area. If most of the water is found on the other side if the seedlings has enough energy it will grow roots seeking water and nutrients. So, the roots would grow that direction.
If we assume all factors are the same through all of the pot. And the seedlings' roots are not bound, then it will only develop root where it can. If it can't grow that direction it will grow more the other direction. There are plants like bamboo that will send shoots down to go under an obstacle, even growing outside the bottom of the pot trying to spread. And grow equally the other direction as well,if it can. If it can't then it will only develop roots where there is adequate space provided.